The present invention relates to a method of preventing breakage of insulated wire and momentary interruption caused by a lightening surge in the neighborhood of an insulator in a high voltage overhead distribution system.
The mechanism which causes breakage of insulated wire is well known. Specifically, that when an insulating cover in the neighborhood of an insulator is punctured by a lightening surge, multiphase flashover is followed by AC follow current through a metallic arm fixing insulator and the conductor of the insulated wire is vaporized by arcing heat as a result of AC short circuit current flowing concentrically to abovementioned punctured portion, thereby causing wire breakage. To prevent such insulated wire breakage, it is important that AC follow current occurring along a discharge circuit is cut off after the discharge circuit is formed by a lightening surge.
To solve these problems, applicants have already proposed, as disclosed, for example, in Japanese Patent Application No. Hei 8-171709, a method of preventing insulated wire breakage and momentary interruption by a lightening surge in an overhead distribution system including insulated wire, in which the conductor of said insulated wire is directly or indirectly exposed at a point spaced from a supported point of the insulated wire by an insulator in a predetermined length, whereby it is possible for creeping flashover to occur by a lightening surge but impossible for AC follow current to follow. This method has the effect that creeping discharge over the outer surface of insulated wire is not followed by an AC short circuit and wire breakage and momentary interruption are not caused when an insulation breakdown point of an insulated wire cover is separated from an insulator 75 cm apart in the case of a 6.6 kV overhead distribution system.
This method, nevertheless, requires research about the thickness of the insulating cover and the like, as the insulating cover is frequently punctured before creeping discharge occurs in the case of applied high voltage.
Thus, an object of the present invention is to provide a method in accordance with which creeping discharge is induced with certainty without puncture of insulated wire after determination of the creeping discharge characteristic of the insulated wire.
To solve the aforementioned problem, a method of preventing insulated wire breakage and momentary interruption according to this invention is characterized in that an overhead distribution system including insulated wire is provided with insulated wire for field relaxation having twice such a predetermined length, that creeping flashover occurs as a result of a lightening surge from a supported point of said insulated wire by an insulator but is not followed by AC follow current, and said insulated wire for field relaxation is disposed in parallel to said insulated wire, supported at its central portion by said insulator, connected at both ends of the conductor of said insulated wire for field relaxation to the conductor of said insulated wire and covered with an insulating cover at their connected portion.
Another method of preventing insulated wire breakage and momentary interruption according to this invention is also characterized in that overhead distribution system including insulated wire is provided with insulated wire for a grounded side back electrode having such a predetermined length that creeping flashover occurs as a result of a lightening surge from a supported point of said insulated wire by an insulator but is not followed by AC follow current, and said insulated wire for the grounded side back electrode is disposed in parallel to said insulated wire, one non-insulated end of the conductor of said insulated wire for grounded side back electrode being connected to a grounded terminal of said insulator and another insulated end of the conductor of said insulated wire for the grounded side back electrode from discharging electrode disposed on said insulated wire is insulated with insulating material.